Methionine in proteins may be subject to attack by unavoidable reactive oxygen species produced as side products of electron transport process in the cell, forming methionine sulfoxide. An enzyme, methionine sulfoxide reductase, can reverse this oxidation. A method of assay of the reductase has been modified and adapted to measure the activity in rat liver homogenates from animals of various ages. It was found that the specific activity of the reductase gradually dropped so that levels at 24 months were about 80 percent of those found at two months in Fisher 344 rats. This observation parallels that made earlier for carbonyl reductase using metyrapone as substrate. The reductase has been purified from Escherichia coli